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Repo of the Day

cosscom/coss

Published: Mar 28, 2025

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Origin UI is an extensive collection of copy-and-paste components for quickly building application UIs. Powered by Tailwind CSS and React.

Summary

coss.com/ui is the official design system for Cal.com, packaged as a Turborepo monorepo that contains the coss ui component library, its documentation site, and a legacy Origin UI snapshot. The coss ui components sit on Base UI primitives and are styled with Tailwind CSS, following a copy-and-own model inspired by shadcn/ui. Everything in the repo is TypeScript, with shared linting and formatting handled by Biome.

What it is useful for

The repository is useful for React engineers who want accessible, pre-styled components without adopting a large runtime library. The coss ui code in apps/ui/ is meant to be copied into your own project and then customized. The apps/origin/ directory retains a pre-acquisition collection of Radix-based, shadcn-style components, though the README flags it as legacy with "limited support and maintenance." The monorepo also exposes packages/ui/ for shared components and packages/typescript-config/ for shared TypeScript settings, useful as a reference for organizing a multi-app TypeScript codebase that uses Next.js, TanStack Table, and Tailwind.

How engineers can use it

The README documents Bun as the package manager. The local workflow is:

  1. Create .env.local files in each app directory. For apps/www/, set NEXT_PUBLIC_APP_URL=http://localhost:3000 and NEXT_PUBLIC_COSS_UI_URL=http://localhost:4000/ui. The apps/ui/ and apps/origin/ apps need similar variables pointing at each other's local URLs.
  2. Run bun run dev to start all apps, or bun run dev --filter=ui (or --filter=www, --filter=origin) for a single app.
  3. Run bun run build with the same --filter syntax to build all apps or a specific one.

Turbo watches .env* files and invalidates the cache when they change. Biome is used for linting and formatting, and every package is 100% TypeScript.

A documented limitation is the mixed license: apps/origin/ and apps/ui/ are MIT, but every other directory is AGPLv3, so engineers embedding these components in proprietary products should review LICENSING.md first. The Origin UI code is explicitly marked as legacy, so new work should target the coss ui primitives instead. Contributing guidelines live at apps/ui/CONTRIBUTING.md.